Each entry consists of a term (in bold), a definition of the term, and a number in parentheses (0) indicating the source of the definition. Clicking on the speaker icon will give you the pronunciation of the term. (Be sure to close the window for the sound player after each use.) The pencil icon will give you a drawing, and the camera icon will give you a photograph.

obligate. Restricted to a particular set of environmental conditions, without which an organism cannot survive. (e.g., an obligate parasite can survive only by parasitizing another organism.) (20)

obligate anaerobe. An organism that can grow only under anaerobic conditions. (16)

obligate parasite. An organism that is incapable of living as a saprophyte and must live as a parasite. (15)

oblong. (Of spores) longer than broad (about twice as long or somewhat less), with sides nearly parallel and with ends more or less flattened. (17)

obovate. Egg-shaped, with the wide end outward. (5)

obovoid. Egg-shaped, with the narrow end outward. (5)

obtuse. (Of pileus, cystidia, spores) rounded or blunt. (17)

odontostylet. See stylet. Synonymous with onchiostylet. (14)

oedema. Intumescence or blister formation because of an increase in intercellular water. (Also edema.) (20)

ontogeny. Development of the individual.

onchiostylet. In nematodes: A stylet developed from a special cell in the anterior part of the esophagus from which it moves into place during each molt. (17)

oocyst. Synonymous with oogonium. (17)

oocyte. Female germ cell. (14)

oogamy. A type of heterogamy in which plasmogamy takes place between a large nonmotile egg and a small motile male gamete or cytoplasm from an antheridium. (15)

oogonium. A female gametangium that contains one or more discrete gametes. (Pl. oogonia.) (15)

Oomycetes. A class of aquatic and terrestrial fungi (subdivision Mastigomycotina) that typically produce oogonia and zoosporangia in which form zoospores having one anteriorly-directed tinsel flagellum and one posteriorly-directed whiplash flagellum.

oomycete. A member of the Oomycetes. (16)

oosphere. A large, naked, nonmotile, usually spherical, female gamete. (15)

oospore. A thick-walled spore that develops from an oosphere through plasmogamy or parthenogenesis. (15)

operculate. Of an ascus or sporangium, opening by a subcircular apical lid to discharge spores, as in asci of the Pezizales. (8)

operculum. A flap or lid-like covering over the opening of an ascus or sporangium. (Pl. opercula.) (15)

opisthodelphic. Of nematodes: Having the uterus (or uteri) directed posteriorly. (14)

ostiole. 1. A neck-like structure in an ascocarp, lined with pariphyses, and terminating in a pore. 2. The opening of a pycnidium. (23)

ostiolum. See ostiole. (Pl. ostiola.)

ovary. Female sexual gland in which the ova, or eggs, are formed. (14)

oviduct. A short, usually tubular, thick-walled part of the female reproductive system between the ovary and spermatheca. (14)

oviparous. Producing eggs that hatch after expulsion from the body. (14)

ovoid. Egg-shaped. (17)

ovoviviparous. Producing eggs that hatch within the body. (14)